Yes—I should have said an issue with the GR backend, not necessarily GR per se.

Presumably what’s happening is that the color :transparent—which is an RGBA black with an alpha value of zero—is being passed to GR in such a way that the alpha value gets lost. I suppose this means that Plots.jl isn’t adhering to the GR interface for fill color opacities, at least when it comes to the background color.